JMCMAN Posted August 12, 2007 Share Posted August 12, 2007 I have isolated my problem to my amp. With nothing connected to my amp ( no pre amp ) I get a hum (while tubes warm up 10- 20 seconds) that turns into a midrange sounding tone that is constant. I have checked all the tubes over and replaced two weak tubes ( 12AX7, 12AU7) one of the output tubes shows weakness in the life or filament test but reads okay otherwise. The sound does not change when I turn the output gains down all the way. I could use some advise as to where to start looking for a problem.Jim Quote Link to comment Share on other sites More sharing options...
mikebse2a3 Posted August 12, 2007 Share Posted August 12, 2007 I have isolated my problem to my amp. With nothing connected to my amp ( no pre amp ) I get a hum (while tubes warm up 10- 20 seconds) that turns into a midrange sounding tone that is constant. I have checked all the tubes over and replaced two weak tubes ( 12AX7, 12AU7) one of the output tubes shows weakness in the life or filament test but reads okay otherwise. The sound does not change when I turn the output gains down all the way. I could use some advise as to where to start looking for a problem.Jim Hi Jim If the sound is in both channels my first place to look at would be the possibility of a bad filter capacitor in the power supply of the amplifier. Not certain but most likely one of the electrolytic capacitors or a ground problem in the power supply is at play here. mike tn[] Quote Link to comment Share on other sites More sharing options...
JMCMAN Posted August 12, 2007 Author Share Posted August 12, 2007 Thanks Mike, the sound is coming from both channels. Any safety precautions needed when checking filters? Jim Quote Link to comment Share on other sites More sharing options...
mikebse2a3 Posted August 12, 2007 Share Posted August 12, 2007 Thanks Mike, the sound is coming from both channels. Any safety precautions needed when checking filters? Jim Yes! The main DC voltages in common tube amplifiers can range from 300v to 600v and can be lethal if you don't know what your doing. Most amplifiers DC voltages will bleed off within a few minutes of turn off but depending on design some can retain voltages for many hours or days. mike tn [] Quote Link to comment Share on other sites More sharing options...
sparrow Posted August 25, 2007 Share Posted August 25, 2007 It is a filter cap problem. Replace the caps and amp will be like new again! Quote Link to comment Share on other sites More sharing options...
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